Magnetic domain devices are being used primarily as sequential-access memory devices in which information is stored in digital form. Functionally, such memory devices may typically comprise a number of shift registers ("minor loops") each of which is in communication with a common, "major loop" shift register. Information is represented by the presence or absence of magnetic domains in a layer of magnetic material, and the function of shifting is effected by means of a magnetic field which may be controlled as to strength and direction.
In a preferred device structure magnetic domains are nucleated and propagated in a layer of magnetic garnet material which is deposited on a nonmagnetic garnet substrate having lattice parameters which are compatible with those of the magnetic layer. Typically, the substrate is made of gadolinium-gallium garnet material and the magnetic layer is patterned after yttrium-iron garnet, Y.sub.3 Fe.sub.5 O.sub.12 (YIG). Magnetic layers of desirable composition and quality can be deposited on a gadolinium-gallium substrate by a process known as liquid phase epitaxy (LPE) as generally described, e.g., by M. H. Randles, "Liquid Phase Epitaxial Growth of Magnetic Garnets", Crystals for Magnetic Applications, Springer, 1978, pp. 71-96.
Magnetic domain propagation paths may be defined in a layer of magnetic material, e.g., by a patterned overlying metallic layer or by a pattern resulting from selective ion implantation. Representative of the latter approach are designs as disclosed, e.g., in U.S. Pat. No. 4,249,249, issued Feb. 3, 1981 to P. I. Bonyhard et al. and in the paper by T. J. Nelson et al., "Design of Bubble Device Elements Employing Ion-Implanted Propagation Patterns", Bell System Technical Journal, Vol. 59 (1980), pp. 229-257.
Desired magnetic anisotropy in a layer of magnetic material results in an "easy direction" of magnetization which is perpendicular to the layer and which renders the layer capable of sustaining magnetic domains whose magnetization is antiparallel to the magnetization of layer material surrounding the domains. Magnetic anisotropy may be "strain induced" as understood to be due to an appropriate disparity between crystallographic lattice dimensions of supported layer and substrate. Alternatively, anisotropy may be "growth induced" as considered to be due to local strain or preferential ordering realized upon deposition of a material in which a crystallographic site such as, e.g., the dodecahedral site is occupied by a mixed ion population. This distinction is made, e.g., in U.S. Pat. No. 3,886,533, issued to W. A. Bonner et al. on May 27, 1975.
Continuing development effort is aimed primarily at reducing magnetic domain size while maintaining or increasing domain wall mobility. Considerable progress has been made towards fast devices having high packing density as illustrated, e.g., by D. J. Breed et al., "Garnet Films for Micron and Submicron Magnetic Bubbles with Low Damping Constants", Applied Physics, Vol. 24 (1981), pp. 163-167, and by J. M. Robertson et al., "Garnet Compositions for Submicron Bubbles with Low Damping Constants", Journal of Applied Physics, Vol. 52 (1981), pp. 2338-2340.
Recently, a need has arisen for devices to operate in inhospitable environments such as, e.g., aboard space stations where operation may be required over a wide temperature range and, in particular, over a range which extends to very low temperatures.